Archived posting to the Leica Users Group, 2006/01/20
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Hi Douglas, It is true that very few metals are the used as the pure element, AFAIK, and certainly the metals I use, are all alloys. Ally wheels for cars are mainly aluminium, though there is a small percentage of magnesium and other alloying elements. Ferrari use magnesium wheels on their road cars but they have to be immediately surface treated and repainted to avoid corrosion if scratched. "Ally" wheels corrode readily if used on salt de-iced roads, unless painted, but not as fast or badly as magnesium. Here is an excerpt from a materials handbook, on the engineering use of magnesium alloys. "Magnesium, with a density of ?1.7gcm-3 is significantly lighter than most other structural metals. The mechanical properties of the pure metal are however very poor such that only alloys of magnesium are of practical value in engineering applications. Under ?ordinary? atmospheric conditions magnesium and its alloys have a good resistance to corrosion due to the formation of a passive oxide layer as happens with aluminium and titanium. Unfortunately this layer is not completely impervious particularly in the presence of damp air containing salts. Corrosion resistance can thus be quite poor under adverse conditions and care must be taken when selecting magnesium based alloys for certain applications. The low density of the base metal ensures that the alloys are also extremely light. Aluminium alloys are significantly stronger than magnesium alloys but the low density of the latter affords higher specific strength and stiffness. Magnesium alloys also possess good machinability and can be readily welded using TIG. Having said that they are extremely soft and a ?bulk? penalty must be paid as a large volume of material is required to achieve equivalent properties to other structural metals. Furthermore they are expensive and their resistance to corrosion is not as good as that of aluminium alloys. There are three primary groupings of magnesium alloys in common use: casting alloys, wrought alloys and those which can be used in both ways. For example; 1. Magnesium-aluminium-zinc alloys may be precipitation hardened and are used in casting, extrusions and forging. 2. Magnesium-manganese alloys are readily weldable and mainly used in sheet fabrication processes. 3. Magnesium-zirconium-rare earth alloys can be used in both cast and wrought forms. They have reasonably high proof stresses, resistance to impact and corrosion and may be easily fabricated." The alloys used on cameras will be type 1, the others would be too expensive. Many metals which corrode form a thin passive layer of oxide on their surface almost immediately, this prevents further obvious corrosion. On aluminium and titanium the oxide layer is thin enough to be "invisible". On brass it tarnishes then stabilises, which makes it good for engineering but tiresome for shiny ornaments! Basically magnesium is "the new titanium" in the camera marketing game. It is nowhere near as good as titanium but massively cheaper and easier to manufacture from. It is an unfortunate fashion which will hopefully not last long. Frank On 20 Jan, 2006, at 00:55, Douglas Sharp wrote: > I was led to believe that the so called magnesium used was, in fact > always a magnesium-aluminium alloy. As Almag - used for car wheels > etc - it doesn't seem to be all that susceptible to corrosion > Douglas > > John Lee wrote: > >> The top plate of the Leica R9 is made from magnesium. >> >> John >> >> On 1/19/06, Bill Marshall <billgem@hotmail.com> wrote: >> >>> Some final thoughts on the recent discussion of the uxe of >>> magnesium plating >>> on the ZI . . . >>> >>> Claims were that magnesium is corrosive, which is true. Recent >>> development >>> of magnesium alloys have largely eliminated this problem. Some >>> still don't >>> like magnesium. Okay. >>> >>> Let's turn the question around. What about brass? Corrosive? >>> >>> Well, it's certainly not water proof. Brass is also an alloy - >>> copper & >>> zinc. Copper is highly corrosive. It will rust with exposure to >>> even small >>> amounts of water within a week. Zinc is also corrosive unless it is >>> electroplated. What metal is closest to zinc in chemical >>> composition? >>> Magnesium. With proper electroplating, magnesium will be about as >>> resistant >>> to corrosion as zinc. >>> >>> Bottom line is whether these materials will fare well in their >>> intended >>> applications, not in an abstract discussion of metallurgy. For its >>> application as external cladding on a camera body, magnesium like >>> brass >>> (copper & zinc) will do just fine - unless someone tries to use >>> it as an >>> underwater camera. Leica agrees with this point, as evidenced by >>> their use >>> of magnesium on the Digilux & their description of it as "robust & >>> long-lived." >>> >>> Bill >>> >>> >>> >>> _______________________________________________ >>> Leica Users Group. >>> See http://leica-users.org/mailman/listinfo/lug for more information >>> >>> >> >> >> _______________________________________________ >> Leica Users Group. >> See http://leica-users.org/mailman/listinfo/lug for more information >> >> > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information